I largely agree with the overall conclusion of Pielke et al. in their Commentary ‘Dangerous assumptions’ (Nature 452, 531–532; 2008) that the Intergovernmental Panel on Climate Change (IPCC) assessment is overly optimistic, but I fear that the situation is even worse than the authors imply.

Debate about the energy challenge of climate change has ignored several key facts about global energy and its future.

All long-range energy forecasts fail in various ways. The scenarios for 2100 in the 2000 IPCC Special Report on Emission Scenarios are risible: even if one of them were to approximate actual demand, we will remain ignorant of what its regional and sectoral composition would be a century from now — and it is this that will determine emissions of greenhouse gases. Basing policies on computerized fairy tales is inadvisable.

Why argue about plausible rates of future energy-efficiency improvements? We have known for nearly 150 years that, in the long run, efficiency gains translate into higher energy use and hence (unless there is a massive shift to non-carbon energies) into higher CO2 emissions.

The speed of transition from a predominantly fossil-fuelled world to conversions of renewable flows is being grossly overestimated: all energy transitions are multigenerational affairs with their complex infrastructural and learning needs. Their progress cannot substantially be accelerated either by wishful thinking or by government ministers' fiats.

Carbon sequestration is irresponsibly portrayed as an imminently useful large-scale option for solving the challenge. But to sequester just 25% of CO2 emitted in 2005 by large stationary sources of the gas (9.6 Gm3 at the supercritical density of 0.468 g cm−3), we would have to create a system whose annual throughput (by volume) would be slightly more than twice that of the world's crude-oil industry, an undertaking that would take many decades to accomplish.

China, the world's largest emitter of CO2, has no intention of reducing its energy use: from 2000 to 2006 its coal consumption rose by nearly 1.1 billion tonnes and its oil consumption increased by 55%.

Consequently, the rise of atmospheric CO2 above 450 parts per million can be prevented only by an unprecedented (in both severity and duration) depression of the global economy, or by voluntarily adopted and strictly observed limits on absolute energy use. The first is highly probable; the second would be a sapient action, but apparently not for this species.

See also:

Energy assumptions were reasonable at the time, but not now

Future scenarios for emissions need continual adjustment

Climate policies will stimulate technology development

IPCC’s climate-policy assumptions were justified